Mrs Willetts (61), whose involvement with the Oamaru Operatic Society goes back to her teenage years, will be the first on stage, in front of an audience, since the refurbishment of the historic building.
While the opera house will not open until February, the staging of Oklahoma! is part of a shakedown period to evaluate the venue and its systems.
One of the reasons Mrs Willetts auditioned was her desire to be part of the redeveloped building, which looked "absolutely amazing".
"It's all very exciting. I think we are very fortunate to have this in our little centre. The town needs it . . . for future generations."
Mrs Willetts' first role for the society was in the chorus of Pink Champagne in the early 1960s.
Since then, she has had other chorus roles, been a dancer and also played some major parts, including Florence in Chess and the Wicked Witch in The Wizard of Oz.
Despite her years of experience, she still got nervous.
In Oklahoma! , she plays Aunt Eller - "a feisty, loveable character".
It was a "beautiful part".
"To me, she's the lynchpin in the whole show. She's the matriarch of the show," she said.
Her part did not involve a lot of singing, but there were some "nice gutsy little numbers", like The Farmer and the Cowman - "That's beautiful. I just love it".
Mrs Willetts came from the large McLennan family who had always been encouraged to be part of the community, "whatever we're doing, wherever we are".
"I love doing this because it's really being part of the community. It's like being part of a big family."
She was grateful for the support of husband Tony and her family, and she warned any wolf-whistles during the show were likely to come from her relatives.
Her younger daughter, Emma, was coming from New York to see her mother in the show.
"Whatever I do, they just get right behind me," she said.
With a love of being on stage, Mrs Willetts had no thoughts of retirement.
"I don't think this will be the swansong. I don't think it will ever be the end because there's always so much you can do backstage.
"Age is just a number. It's how you feel in your head and I still feel really young and vibrant - at times."
